There are various applications in which there is a need to collect radiation. As one example, a sight or scope is often mounted on a weapon to help a person aim the weapon at an intended target. Many sights superimpose a reticle on the image of the target. It can be advantageous if the reticle is illuminated. Therefore, some existing sights collect ambient radiation, and use it to illuminate the reticle. Devices have previously been developed to collect ambient radiation. Although these devices have been generally adequate for their intended purposes, they have not been completely satisfactory in all respects.